This patent concerns spacers used for the installation and spacing of communications and power cables under the ground and above ground. Cables for electric power and communication lines are run underground in order to protect them from above-ground elements and from the interference and damage they would suffer when installed above the ground or on poles or structures.
Power and communication distribution cables are typically routed aboveground. They are routed underground when for various reasons, aboveground routes are not permitted or are not possible. Most underground power and communication cables for private homes are dug directly. Power and communications cables for mission-critical installations receive more circumspect treatment. These installations include hospitals, airports, military bases, and major electric transmission lines. For most of these installations, an open cut trench is dug, conduit is placed in the lower portion of the trench in an organized and controlled separation bank, and the conduit is then encased in concrete forming what is commonly known as a concrete encased duct bank. The conduit is most often a round pipe made from plastic but on occasion may be of other shapes and material. Typically, the top of the duct bank is two feet or more below grade. The area between the top of the duct bank and grade is backfilled with sand, gravel, soil, or other appropriate fill. Power or communication cables or both are then pulled into the conduits.
In some instances, however, it is not possible to route an open-cut trench directly to the desired location without disruption. It may therefore be necessary to cross under a highway, a railroad, a waterway, or other obstruction. For these installations, a tunnel must be dug underneath, typically by digging a straight bore or by using directional drilling. A straight bore is typically used for relatively straight installations of less than 500 feet (150 m). After the bore is dug, a casing, typically made of steel, is pulled through the bore and conduit with spacers is pulled into the casing. Directional drilling is typically used for longer bores. In these installations, steel or other casing is most often used, but some installations are casingless. The directionally drilled casingless installation method is typically selected when the extra protection offered by a casing and grout is not deemed necessary and economy of the installation is of prime importance.
Underground conduits typically are placed in casings made of steel, high-density polyethylene (HDPE), concrete, fiberglass-reinforced thermoset polymers, or centrifugally-cast fiberglass reinforced polymers. Other casing materials may also be used. Casing lengths may range from 10 feet to 3,000 feet (3 to 920 m) or longer, with diameter from 4 inches to 60 inches (10 to 150 cm), or larger. The conduits themselves are typically made from high density polyethylene (HDPE), polyvinyl chloride (PVC), fiberglass reinforced epoxy. Other conduit materials may be used. Conduit-in-casing installations are used to route communications and power cables under highways, streams and rivers, railroad track, and other obstructions that, for one reason or another, may not be disturbed. Underground power and communications cables are typically placed in directionally-drilled tunnels or straight-bored tunnels.
Directionally drilled holes normally used for conduit-in-casing installations or casingless installations refer to a tunnel that starts at grade or in a pit that is slightly below grade. The tunnel goes downward at approximately a 20° angle until it is low enough to go under the obstruction. The obstruction may be 100 feet (30 m) or more below grade. When the tunnel is low enough to go under the obstruction it turns gently and then follows a line parallel to grade. When the tunnel has cleared the obstruction, it sweeps upward at an angle, typically about 40°, and exits at grade.
In order to prepare a directionally drilled hole, construction crews start first with a pilot hole. After the pilot hole is installed, reamers of successively larger size are pulled through the hole until the hole is approximately 50% larger than required for the duct bank or casing. As the reamers are pulled through, the hole is kept full of mud made with Bentonite to keep the tunnel from caving in or filling with water. Bentonite is a natural clay found in the earth's strata. After the directionally bored hole is completed, the duct bank is pulled into place displacing part of the Bentonite.
Straight bores may be prepared in many ways, but are most often accomplished as follows. A boring pit is dug on one side of the obstruction, and a receiving pit on the other side. A length of auger is placed inside steel casing having a similar length and slightly large outer diameter. The auger and casing are placed into the boring pit. Using a special purpose boring machine, which is usually track-mounted, the casing is hydraulically jacked in the direction of the receiving pit while rotating the auger to remove the earth from inside the casing. Successive lengths of casing are welded to each other and successive lengths of auger are attached to remove earth from the casing. When the casing and auger reach the receiving pit, the augers are removed.
Meanwhile, an assembly of the conduits is prepared, the assembly including the conduits and spacers to maintain separation of the conduit in the casing. The spacers are placed along every several feet of conduit length. The assembly of conduits and spacers is then pulled into the casing and grout is placed between the casing and the conduit, filling the space in between them. Grout is a fluid mixture of sand, cement and water. Special additives are sometimes used to make the grout very fluid, to enhance thermal conductivity, or to slow hydration or curing of the grout. After the grout has hydrated, power and communications cables are pulled through the conduits.
One technique used to fill the space is known as the sacrificial grout injection pipe technique. This technique requires multiple sacrificial grout injection pipes or hoses, normally made from 2 inch or 3 inch diameter HDPE or PVC, that are successively placed along the length of the duct bank. Grout is pumped into the end of each grout pipe in turn until the space in the casing is filled with grout over the full length of the casing. When the space that is reached by one grout pipe is filled, the next pipe or hose is used until the entire space in the casing has been filled with grout.
A number of methods have been devised for organization and separation control of conduits for open cut trench concrete encased duct banks. Many of these methods are depicted in the following patents: U.S. Pat. Nos. 2,462,399; 2,686,643; 2,937,833; 3,523,667; 3,643,005; 3,856,246; 3,964,707; 4,183,484; 4,244,542; 4,306,697; 4,601,447; 4,618,114; 5,104,072; and 5,605,419. These prior art spacers and concrete encased duct bank installation methods hold the conduits vertically and horizontally, but do not provide for longitudinal restraint, probably because these patents envision filling an open cut trench with concrete or grout from above, not from the side as would be the case in a closed casing or directionally-bored hole.
U.S. Pat. Nos. 5,137,306, 5,372,388, 6,076,863 and 6,711,328 depict conduits separated by spacers that are placed inside a casing. These patents related to very small conduits for fiber optic cables. Since fiber optic cables do not generate any heat, so no provision is made in these designs for placement of grout between the conduit outer diameter and the casing inner diameter.
What is needed is a better way of spacing and holding apart conduits for power and communications cables in underground or confined installations. The improved method should allow for controlled spacing and excellent heat conduction, while providing an efficient, economical, and easy way to install the conduits.